Add PNG screenshot support

Saves client version, map, and player info in PNG text fields.
Merge remote-tracking branch 'spearmint/master'

Conflicts:
	code/client/cl_main.c
	code/client/client.h
	code/qcommon/common.c
	code/renderercommon/tr_image_png.c
	code/renderercommon/tr_public.h
	code/renderergl1/tr_init.c
	code/renderergl1/tr_local.h
Add zlib files for compressing PNG screenshots
Fix levelshot command

Needs to be sent to client connection, otherwise lc# is added to beginning of command. Client doesn't catch "lc0 clientLevelshot".
Support saving levelshots as PNG and JPEG

Also supports changing levelshot size, example "levelshot 256" or "screenshotJPEG levelshot 256"
Add levelshot <size> to README
Merge remote-tracking branch 'spearmint/master'

Conflicts:
	code/renderergl1/tr_init.c
Merge remote-tracking branch 'ebx/master'